WO2005043130A1

WO2005043130A1 - Device and method for determining the gas content of a liquid

Info

Publication number: WO2005043130A1
Application number: PCT/EP2004/012048
Authority: WO
Inventors: Christian Beck; Bolko Raffel; Hubert Ehbing; Peter Jähn; Franz Schmitt
Original assignee: Bayer Materialscience Ag
Priority date: 2003-10-30
Filing date: 2004-10-26
Publication date: 2005-05-12
Also published as: US20050092062A1; DE10350757A1

Abstract

The invention relates to a device for determining the gas content of a liquid, said device at least comprising a housing (10), a measuring chamber bottom (23) provided with a cavity (24), and a measuring chamber top (25) located inside the housing (10). Said measuring chamber bottom (23) and measuring chamber top (25) are respectively mounted in such a way that they can be independently displaced along the longitudinal axis of the housing (10) by means of a piston (21, 22). Furthermore, the measuring chamber bottom (23) and the measuring chamber top (25) form a measuring chamber (26, 26') in the housing (10). The measuring chamber bottom (23) or the measuring chamber top (25) is connected to a pressure sensor (30), and the housing (10) comprises a liquid supply line (16) and a liquid discharge line (17).

Description

Device and method for determining the gas content of a liquid

The invention relates to a device and a method for determining the gas content of a liquid.

In plastics processing technology, it is often necessary to determine the gas content of liquids, e.g. to determine the gas content of liquid plastic components for foam production, in order to be able to work with a constant and known gas content in ongoing work or production operations. For the determination of the gas content of liquids, in particular liquid plastic components, the state of the art, e.g. DE-A 37 20 904, WO 99/02963, numerous measuring devices are known. The known measuring devices generally use measuring cylinders provided with measuring pistons, into which a certain sample quantity from the system carrying the gas-laden liquid is introduced at intervals. This is generally done in such a way that the sample quantity loaded with the dissolved and possibly also free gas is exposed to a negative pressure in the closed measuring cylinder space by increasing the volume of the measuring cylinder space by appropriate piston adjustment. This releases the gas in the sample amount. From the determined changes in volume and pressure of the measurement sample, the gas loading of the liquid can be calculated according to the known physical relationships (gas law). The same is possible if the measurement sample in the measuring cylinder is measured by compression using the measuring piston and / or by a combination of decompression and subsequent compression with pressure measurement under the various test conditions and simultaneous volume determination of the measuring space at the different piston positions.

In general, however, these methods cannot readily be used in a fast online measuring device, since they either cannot be operated in a bypass arrangement in the flow and / or are associated with considerable structural outlay. Furthermore, in the known measuring devices, a relatively large amount of sample is necessary, as a result of which a relatively long measuring time is required. Often there is also no possibility of tempering the sample, which also leads to a relatively long measuring time. Finally, cleaning steps are necessary between the individual measurements in the known devices. This makes a quick and cyclical measurement difficult or even impossible.

The object of the present invention is to provide a device and a method with which the gas content of liquids can be determined online and relatively quickly. The device should be constructed as simply as possible, ie the construction effort should be as low as possible. The invention relates to a device for determining the gas content of a liquid, at least consisting of a housing, a measuring chamber floor with a cavity and a measuring chamber ceiling inside the housing, the measuring chamber floor and the measuring chamber ceiling each being movably supported independently of one another along the longitudinal axis of the housing by means of a piston are, the measuring chamber floor and the measuring chamber ceiling form a measuring chamber in the housing, the measuring chamber floor or the measuring chamber ceiling is connected to a pressure sensor and the housing has a liquid supply and a liquid discharge.

The device according to the invention consists of a, preferably cylindrical, double-piston system. Both pistons are movably mounted along the longitudinal axis of the, preferably cylindrical, housing. Both pistons are independent of each other, e.g. pneumatically via a throttle system, movable. In operation, the device is arranged vertically. Accordingly, the pistons are also referred to below as lower and upper pistons. A measuring chamber base is arranged at the upper end of the piston rod of the lower piston. A measuring chamber ceiling is arranged at the lower end of the piston rod of the upper piston. The space between the measuring chamber floor and the measuring chamber ceiling forms the measuring room. The measuring chamber floor has a cavity on its surface facing the measuring chamber ceiling. The measuring chamber ceiling or the measuring chamber floor is connected to a pressure sensor. The measuring chamber ceiling is preferably connected to a pressure sensor. The measuring chamber floor and / or the measuring chamber ceiling and / or the housing is connected to a heating source, e.g. a heating coil or a heating jacket. The measuring chamber floor is preferably connected to a heating source.

Liquid flows into the housing through a liquid supply and out of the housing through a liquid discharge. The liquid supply and the liquid discharge are arranged in alignment, for example. They can also be arranged at any angle to one another and / or in different planes transverse to the longitudinal direction of the housing.

The cavity in the bottom of the measuring chamber serves to receive the liquid to be sampled, which is supplied through the liquid supply. The cavity in the bottom of the measuring chamber is designed so that the liquid to be tested forms a thin film. The dimension of the cavity is characterized by a ratio of volume to thickness of 10 ⁶ : 1 to 1: 1, preferably 10: 1 to 1: 1. The geometric shape of the cavity can be chosen arbitrarily. A cylindrical cavity is preferred.

The cross-sectional area of the cavity in the measuring chamber floor can be arbitrary. The cavity preferably occupies the largest possible area of the measuring chamber floor, so that the Liquid to be tested forms as large a surface as possible in the cavity. For example, the cavity can occupy almost the entire surface of the measuring chamber floor, with only a narrow edge around the cavity, for example of the order of a few millimeters, is present. For example, in a measuring chamber base with a diameter of 50 mm, the cavity can have a diameter of 48 mm.

The surface of the cavity, i.e. the inner wall of the cavity can be flat. To increase their surface area, the. However, the cavity also has a structured surface made of knobs, grooves or the like. exhibit.

Another object of the invention is a method for determining the gas content of a liquid using the device according to the invention with the following steps:

a) with the aid of the pistons, the measuring chamber floor is positioned below the liquid supply and liquid discharge and the measuring chamber ceiling above the liquid supply and liquid discharge such that a liquid flows through the liquid supply into the measuring chamber between the measuring chamber floor and measuring chamber ceiling and through the liquid discharge from the measuring chamber,

b) with the aid of the piston, the measuring chamber floor is moved towards the measuring chamber ceiling until the measuring chamber floor contacts the measuring chamber ceiling,

c) the measuring chamber ceiling is moved away from the measuring chamber floor with the aid of the piston, the measuring chamber being formed between the measuring chamber floor and the measuring chamber ceiling, and the pressure in the measuring chamber being measured with the aid of the pressure sensor,

wherein the sequence of steps a) to c) is carried out at least once.

In the first step a) of the method according to the invention, the measuring chamber base and the measuring chamber ceiling are positioned with the aid of the pistons so that the liquid to be tested flows through the device. On the one hand, this means that the measuring chamber floor and the measuring chamber ceiling are kept at a distance, so that they form a measuring chamber in the housing. On the other hand, the measuring chamber floor and the measuring chamber ceiling are positioned in the housing in such a way that the liquid supply and removal is located between them. In this way, the liquid to be tested can flow through the measuring chamber. The measuring chamber forms the area through which the device flows. If liquid flows through the measuring chamber, the cavity in the measuring chamber floor is also filled with the liquid. In the second step b) of the method, a sample amount is discharged by moving the lower piston with the measuring chamber bottom up, ie in the direction of the upper piston with the measuring chamber ceiling. The measuring chamber floor is moved upwards until the mutually facing surfaces of the measuring chamber floor and the measuring chamber ceiling come into contact and lie on one another. Liquid that is in the measuring chamber is displaced until only the amount remaining in the cavity remains. In this second step, the amount of liquid to be sampled in the cavity must be isolated from the liquid flow that flows through the housing via the liquid supply and discharge. The cavity must therefore be led out of the area of the liquid supply and discharge, ie the flow area, so far that it is located above the liquid supply and discharge. This can happen, for example, in that the lower piston pushes the upper piston so far out of the flow area that the sample is separated from the flow area of the sensor by means of seals on the piston and is ready for measurement. In the third method step c), the gas content is measured in the amount of liquid in the cavity. In order to generate a vacuum, the measuring chamber ceiling is moved upwards by means of the upper piston, ie moved away from the measuring chamber floor. A measuring chamber is again formed between the measuring chamber floor and the measuring chamber ceiling. Due to the vacuum, the gases contained in the liquid escape and evaporate low boiling ^'components until an equilibrium vapor pressure established in the measurement chamber. The evaporation can be supported by a heating source connected to the measuring chamber floor and / or the measuring chamber ceiling and / or the housing. In process step c), the pressure in the measuring chamber is measured. The pressure is measured with the aid of the pressure sensor, which is connected to the measuring chamber ceiling or the measuring chamber floor, for example, continuously during the movement of the piston depending on the piston travel. Alternatively, the pressure measurement can also take place at the end of the piston movement, ie at the end of the evaporation process. With the help of the measured pressure in the measuring chamber, the gas content of the sample or the vapor pressure of the liquid is calculated.

The movement of the upper piston in step c) to generate the negative pressure in the measuring chamber preferably takes place in the range from 1 ms to 1 min. In particular, the duration of the piston movement is in the range of milliseconds, particularly preferably in the range of 0.1 to 4 s.

The sequence of steps a) to c) is carried out at least once according to the invention. The sequence of steps a) to c) is carried out several times in that the sequence of steps a) to c) begins again after step c). After measuring the pressure according to step c), the measuring chamber bottom and the The measuring chamber ceiling is brought back to the starting position according to step a) by means of the pistons. The amount of sample in the cavity is returned to the liquid flow. The method according to the invention is therefore suitable as a cyclic method for the continuous online measurement of the gas content of a liquid.

An advantage of the device according to the invention is that it is comparatively simple, i.e. the construction effort is comparatively low. Sampling is carried out using the cavity. A valve system or gravimetric system for dosing the liquid to be sampled is not required. Since the device has no valves in contact with the product, no malfunctions can occur as a result of clogging or leakage. The pistons can be operated pneumatically so that no servomotors are required. The device also works without additional peripheral devices such as e.g. Circulation pump or vacuum pump. The device according to the invention requires neither a pump to provide an evacuated gas space, nor a pump to supply the liquid to be sampled to the measuring device. A defined vacuum in the measuring chamber is generated by a hydraulically driven piston movement.

Another advantage is the comparatively fast measurement. The duration of a measurement is usually a maximum of 5 minutes. The duration of a measurement cycle is preferably 1 to 5 minutes. The duration of a measuring cycle essentially depends on how quickly the liquid to be sampled evaporates or how quickly the equilibrium pressure is established. Accordingly, a measuring cycle can take more than 5 minutes. The quick measurement is made possible by the rapid degassing of the liquid to be tested in the cavity, since the cavity forms a thin film of the sample liquid. The outgassing time can be further reduced by thermal and or ultrasound support.

Finally, the device advantageously works almost product-free and emission-free because the sample amount is fed back into the liquid flow after the measurement. The device according to the invention is quasi self-cleaning, since after the measurement, the measuring chamber floor and the measuring chamber ceiling are brought back into the starting position and liquid flows through the measuring chamber again. The inflowing liquid washes the liquid sample of the previous measurement from the measuring chamber. The seals on the measuring chamber ceiling and the measuring chamber floor wipe off any liquid drops adhering to the inner wall of the housing. The liquid sample therefore does not have to be removed from the measuring chamber by a separate process step before a new measurement can be carried out. In addition, no additional cleaning step between two measurements is necessary. The device according to the invention can be used, for example, for online measurement. Use as a mobile hand-held device is also possible, for example for determining the gas loading of liquid plastic components or the vapor pressures of liquids which are located, for example, in storage containers.

If the device according to the invention or the method according to the invention for determining the gas content in a flowing liquid is to be used, the measurement can be carried out without interrupting the flow of liquid, for example, by sensing part of the flow via a bypass to which the device according to the invention is connected is.

The device according to the invention is suitable e.g. to determine the proportion of dissolved gases in a Polyetheφolyol for the production of polyurethane foams. After the polymerization reaction of the starting materials, the gas content of the polyether polyol is determined using the device according to the invention. The gas content is calculated from the equilibrium pressure, which is recorded by the pressure sensor, for example by means of the ideal gas law.

Furthermore, the device according to the invention is suitable for determining vapor pressures of liquids in order to check the chemical purity, for example during production.

It is also possible to use the device according to the invention in combination with an appropriate analysis device (TR spectrometer, Raman spectrometer or the like) to qualitatively and quantitatively examine the gas space for its chemical composition.

The invention is explained below with reference to the accompanying drawings. Show it:

Figure 1 shows an embodiment of the device according to the invention in longitudinal section, wherein the measuring chamber bottom and the measuring chamber ceiling according to step a) of the method according to the invention are arranged so that the liquid flows through the device

FIG. 2 shows a section of the device according to the invention according to FIG. 1, the measuring chamber floor and the measuring chamber ceiling according to step b) of the method according to the invention being arranged in such a way that the liquid to be tested is isolated from the liquid flow

3 shows a section of the device according to the invention according to FIG. 1, the measuring chamber floor and the measuring chamber ceiling being arranged in accordance with step c) of the method according to the invention in such a way that the gas content of the liquid to be tested is determined FIG. 1 shows the device 1 according to the invention for determining the gas content of a liquid. It consists of a cylindrical housing 10 with a liquid supply 16 and a liquid discharge 17, so that the liquid 50 to be measured can flow across the housing 10, ie transversely to the longitudinal axis 18 (shown as a dash-dotted line) of the housing 10. In the illustrated embodiment, the liquid supply 16 and the liquid discharge 17 are arranged in alignment. Above the liquid feed 16 and the liquid discharge 17 there is a measuring chamber ceiling 25, below the liquid feed 16 and the liquid discharge 17 there is a measuring chamber floor 23. The measuring chamber floor 23 has a cavity 24 on its upper surface, ie facing the measuring chamber ceiling 25. In the illustrated embodiment, the cavity 24 has a depth of 1 mm and a diameter of 35 mm. The liquid flow 50 across the housing 10 is therefore limited at the top by the measuring chamber ceiling 25 and at the bottom by the measuring chamber floor 23. The space between the measuring chamber ceiling 25 and the measuring chamber floor 23 forms the measuring chamber 26. The measuring chamber ceiling 25 and the measuring chamber floor 23 are sealed off from the housing 10 with elastic seals 13, 14, so that the liquid 50 flowing through is located in the measuring chamber with a short residence time and a small residence time spectrum 26 constantly renewed during operation. A groove for receiving an elastic seal 15 is also provided around the cavity 24.

In the embodiment shown, the measuring chamber ceiling 25 serves to receive a pressure sensor 30. The pressure sensor 30 and the lower, i.e. The surface of the measuring chamber ceiling 25 facing the measuring chamber floor 23 form a plane, so that product deposits and dead spaces are avoided. The measuring chamber base 23 serves to receive a heating element 40. In the lower section of the housing 10 there is an opening 43 through which, for example, the connecting cable 41 of the electrically heated heating element 40 can be connected to a power supply (not shown). In addition, an outer heating element 19, e.g. in the form of a heating jacket. An additional external heating element, which is arranged around the housing 10, can accelerate the evaporation and thus the establishment of equilibrium.

The measuring chamber base 23 is fastened, for example detachably, to the piston rod 27 of the pneumatic piston 21. The measuring chamber ceiling 25 is connected, for example detachably, to the piston rod 28 of the pneumatic piston 22 via a spacer 29. Longitudinal grooves or openings 11 or 12 are provided in the spacer 29 and in the housing 10 in order to lead the cables 31 of the pressure sensor to the outside for a measurement value acquisition (not shown). The position of the measuring chamber ceiling 25 and the measuring chamber bottom 23 shown in FIG. 1 corresponds to the starting position, ie step a), of the method according to the invention. The measuring chamber ceiling 25 is arranged above the liquid supply 16 and the liquid discharge 17, the measuring chamber bottom 23 is arranged below the liquid supply 16 and the liquid discharge 17. The liquid 50 to be sampled flows through the measuring chamber 26 across the housing 10. The cavity 24 is continuously supplied with fresh liquid 50 close to the process while it is flowing through.

The position of the measuring chamber bottom 23 and the measuring chamber ceiling 25 according to step b) is shown in FIG. The piston 21 (FIG. 1) moves the measuring chamber floor 23 against the measuring chamber ceiling 25, so that the two surfaces facing each other come into contact. The excess liquid 50 is displaced between these two surfaces, so that only the cavity 24 is filled with the liquid 50 to be tested. The filled cavity 24 is enclosed by means of the seal 15. The piston 21 moves the measuring chamber floor 23 and the measuring chamber ceiling 25 with the piston 22 (FIG. 1) out of the area of the liquid supply 16 and liquid discharge 17, so that in the end position of the piston 21 according to step b) the seal 14 of the measuring chamber floor 23 above the liquid supply 16 and the seal 14 'is below the liquid supply 16. The amount of sample in the cavity 24 is now completely separated from the liquid flow 50.

FIG. 3 shows the position of the measuring chamber floor 23 and the measuring chamber ceiling 25 according to step c). The piston 22 (FIG. 1) moves upward and removes the surface contact between the measuring chamber floor 23 and the measuring chamber ceiling 25, so that the measuring chamber 26 ', which is evacuated, is formed between the two separate surfaces. Easily volatile components can now evaporate from the sample liquid 50 in the cavity 24 into the evacuated measuring chamber 26 '. The movement of the measuring chamber ceiling 25 by means of pistons 22 takes place in milliseconds. This leads to a sudden evacuation of the measuring chamber 26 '. The pressure in the measuring chamber 26 'changes during the evaporation of the volatile components or gases. The pressure sensor 30 registers the pressure change that approaches a limit value asymptotically. As soon as the limit value is reached, the measurement is considered complete and will be stopped. The cycle of the method according to the invention now begins anew by moving the measuring chamber bottom 23 and the measuring chamber ceiling 25 back into the starting position according to step a) with the aid of the pistons 21 and 22 (FIG. 1). The measured amount of sample from the cavity 24 is displaced from the cavity 24 by the liquid 50 flowing through it and fed to the process. A new measurement of the vapor pressure can thus be carried out with a fresh liquid sample.

Claims

claims

1. Device for determining the gas content of a liquid, at least consisting of a housing (10), a measuring chamber floor (23) with a cavity (24) and a measuring chamber ceiling (25) within the housing (10), the measuring chamber floor (23) and the measuring chamber ceiling (25) is movably supported independently of one another along the longitudinal axis of the housing (10) by means of a piston (21, 22), the measuring chamber floor (23) and the measuring chamber ceiling (25) in the housing (10) a measuring chamber (26, 26 '), the measuring chamber floor (23) or the measuring chamber ceiling (25) is connected to a pressure sensor (30) and the housing (10) has a liquid supply (16) and a liquid discharge (17).

2. Device according to claim 1, characterized in that the measuring chamber floor (23) and / or the measuring chamber ceiling (25) and / or the housing (10) is connected to a heating source (40).

3. Device according to one of claims 1 or 2, characterized in that the cavity has a volume to thickness ratio of 10 ⁶ : 1 to 1: 1, preferably 10 ⁵ : 1 to 1: 1.

4. A method for determining the gas content of a liquid using a device according to one of claims 1-3 with the following steps: a) with the aid of the pistons (21, 22), the measuring chamber bottom (23) below the liquid supply (16) and Liquid discharge (17) and the measuring chamber ceiling (25) positioned above the liquid supply (16) and liquid discharge (17) so that a liquid (50) through the liquid supply (16) into the measuring chamber (26) between the measuring chamber floor (23) and the measuring chamber ceiling ( 25) and through the liquid discharge (17) from the measuring chamber (26) flows b) with the aid of the piston (21) the measuring chamber floor (23) is moved towards the measuring chamber ceiling (25) until the measuring chamber floor (23) the measuring chamber ceiling (25) contacts c) with the aid of the piston (22), the measuring chamber cover (25) is moved away from the measuring chamber floor (23), the measuring chamber (26 ') moving between the measuring chamber floor (23) and the measuring chamber ceiling (25) forms, and with the help of the pressure sensor (30) the pressure in the measuring chamber (26 ') is measured, wherein the sequence of steps a) to c) is carried out at least once.

5. The method according to claim 4, characterized in that the movement of the piston (22) according to step c) takes place within 1 ms to 1 min.